Mixing viscosities doesn't work

JonS · May 9, 2005

I have heard this from some oil companies that mixing doesn't give you what you think.

Here is something i found on google.

"An oil's "weight" is molecular. Additives such as viscosity index improvers
work to give oil different characteristics, but the fact remains. Pick up
any vocational high school textbook, and that is one of the first things
covered concerning oil.

In my teaching days, we used to centrifuge the 30 weight out of the 10
weight after "mixing" just to prove the point.

"Mixing" 10 weight oil with 30 weight oil to get 20 weight oil is like
"mixing" one-inch rocks with three-inch rocks to get two-inch rocks. It
simply doesn't happen.

You will have two quarts of one weight oil and two of another in your
crankcase."

TomH · May 9, 2005

I'm not a tribologist, but the thing about the rocks makes sense, just not sure if oil would perform the same way.

CQUG4R · May 9, 2005

Does this sound like a question for Terry?

I think so.

csandste · May 9, 2005

I'd certainly defer to more knowledgable posters, but...

1. Weights are numeric, not a measure of absolute thickness, i.e. a 20 weight isn't twice as heavy as 10 weight, therefore I doubt if mixing the two would necessarily get you a 15 weight.

2. I would think that the new oil would be somewhere between a 10 and a 20 weight, and I don't think that mixing weights would present problems. My daughters car leaks oil and I occasionally drop in a quart of 20w50 in with her 10w30 as the weather warms up. I doubt if that will do any harm.

dbf · May 9, 2005

State of matter is being ignored in the rock analogy.If the rocks were liquid they likely would combine(mix)just fine.

bulwnkl · May 9, 2005

Yes, the second-level people at Mobil1's tech line have told me the same thing. They said that mixing the 15W50 with the 10W30 was a good thought, but it wouldn't result in an xW40. Rather, it would simply be a mixture of 10W30 and 15W50.

blupupher · May 9, 2005

I would think you would need to look at the cSt of the oil to determine the viscosity. You can have 2 xW-30 oils, and they are both different thickness, one could be on the thin side of 30 and the other on the thick side.
So while I agree with that with a 20 and 40 weight mixed you will not necessarily get a 30 weight, I do not agree that mixing does nothing. The mixing rocks comparison is a totally different thing than mixing oils. For starters, it is a solid vs liquid comparison. If the person saying this can't recognize this and come up with a better example, then I would not give him too much credibility.
But what do I know.

tenderloin · May 9, 2005

If the "rock" example was correct, why would oil manufacturers say their oils are compatible with other oils, synthetic and dino?

With all the different ,manufacturers type of oils weights etc., if you could not safely mix oils (maybe not optimum performance) what a mess we would have.

Bryanccfshr · May 9, 2005

I believe we have seen that blending does change the kinematic viscosity @100°c per some UOA's. The molecular sizes of individual molecules do not change but the efective viscosity should and does change change.

Most formulations we buy off the shelf are a mix of different viscosity base oils to reach a desired viscosity.

rgl · May 9, 2005

If you mix 1in and 2in rocks to make a driveway you will definitely get a different surface than all of one type or the other. I don't think anyone is too concerned with the individual rocks as they would not be with individual molecules, just their combined effect even if not inseparably mixed.

buster · May 9, 2005

This was talked about a long time ago. I believe the analogy was mixing "basket balls with golf balls and baseballs."

427Z06 · May 9, 2005

Viscosity Index Improvers (and other factors) will complicate matters when mixing "finished" oils and then trying to calculate the resultant kinematic viscosity. However, what are tribologist doing when they use blending charts and calculators when mixing base oils? Thus, the rock analogy is a poor one. Staying at a Holiday Inn Express won't make someone a Lubrication Engineer or Tribologist.

I believe that when mixing synthetics of the same brand and type with a limited amount of VII, such as a 10w30 or 20w50, you can estimate to a reasonble degree the resultant 100C kinematic viscosity.

MolaKule can nail this down further.

y_p_w · May 9, 2005

There are so many factors. Base oil type(s), VI improver type(s), additive chemistries. For the most part there likely isn't any catastrophic reaction.

However - I recall corresponding with a lube chemist. One interesting note he brought up were two different detergent additives. When mixed together in equal amounts, the additive mixture thickened the oil such that the two 5W-30 oils were in the 15W-40 range.

Mr Nice · May 9, 2005

Guys, Guys, Guys

Your engine doesn't care if you mix viscosities or Brands. As long as you have the proper amount of oil in the sump.......it will be difficult to kill an engine. I have used and mixed 9 different types of oils (dino, syn blend, synthetic) with ZERO problems.

Al · May 9, 2005

quote:

Originally posted by JonS:
You will have two quarts of one weight oil and two of another in your
crankcase."

That's incorrect. Oil manufactures blend oils of multiple weights to get the final product. And if you mix a quart of 30 wt and 50 wt. you will have 2 quarts in the 40wt range....take it to Vegas and bet on it.

y_p_w · May 9, 2005

quote:

Originally posted by Al:
That's incorrect. Oil manufactures blend oils of multiple weights to get the final product. And if you mix a quart of 30 wt and 50 wt. you will have 2 quarts in the 40wt range....take it to Vegas and bet on it.

Too many variables. You probably will end up with something in between, but there are too many factors to say that will happen 100% of the time.

Lube manufacturers get a 40 weight oil by blending base oils and VI improvers in specific proportions. They don't take their bulk 10W-30 and mix it with their bulk 20W-50 to get a 15W-40.

However - most engines are tolerant of a wide range of oil types, regardless of the warnings of dire consequences carmakers give if you don't use XW-YY weight oil.

JohnBrowning · May 9, 2005

As much as it pains me to say this I am in agreement with 427Z06 100% on this one. His post was almost word for word what I was thinking as I read the opening post for the thread!!

You can mix viscositys just fine. The rock anology very bad not even close! You can get a close enough aproxiamation to make it worth while! You are not going to get an exact number like takeing an average of the two!

Many Corvette owners that like to actualy drive and race their vette's have been mixing for some time. M1 5W30 adn M1 15W50 in a ratio between 85%/15% to about 50%/50% to arive at either a thick 30Wt. to a light 40Wt.!

pscholte · May 9, 2005

quote:

Originally posted by 427Z06:
Staying at a Holiday Inn Express won't make someone a Lubrication Engineer or Tribologist.

Z,

Are you saying this guy has "rocks in his crankcase?"

'00obw · May 9, 2005

Here's a VOA of my 50/50 mix: http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=11;t=000289. The Vis@100 came close to the mathematical average and the from the Viscosity Index Calculation page, http://www.baseoils.shellglobalsolutions.com/calculations/blending_pop.asp, the vis# was 9.7

On a VOA of my 50/50 mix of Mobil 1 0w20 and GC the vis# was 11.32. The mathematical average was 10.25 and from the VIC page was 10.1.

I quess these 2 examples show that mixing 2 different viscosity oils will not always give predictable results.

bgin

bulwnkl · May 9, 2005

quote:

Originally posted by 427Z06:
Staying at a Holiday Inn Express won't make someone a Lubrication Engineer or Tribologist.

Hey, I'm staying at a Holiday Inn Express right now! What DOES it make you?

Mixing viscosities doesn't work

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